powerpc/64s/slb: Fix SLB multihit issue during SLB preload

Overview

In the Linux kernel, the following vulnerability has been resolved:

powerpc/64s/slb: Fix SLB multihit issue during SLB preload

On systems using the hash MMU, there is a software SLB preload cache that mirrors the entries loaded into the hardware SLB buffer. This preload cache is subject to periodic eviction — typically after every 256 context switches — to remove old entry.

To optimize performance, the kernel skips switch_mmu_context() in switch_mm_irqs_off() when the prev and next mm_struct are the same. However, on hash MMU systems, this can lead to inconsistencies between the hardware SLB and the software preload cache.

If an SLB entry for a process is evicted from the software cache on one CPU, and the same process later runs on another CPU without executing switch_mmu_context(), the hardware SLB may retain stale entries. If the kernel then attempts to reload that entry, it can trigger an SLB multi-hit error.

The following timeline shows how stale SLB entries are created and can cause a multi-hit error when a process moves between CPUs without a MMU context switch.

CPU 0 CPU 1

Process P exec swapper/1 load_elf_binary begin_new_exc activate_mm switch_mm_irqs_off switch_mmu_context switch_slb /* * This invalidates all * the entries in the HW * and setup the new HW * SLB entries as per the * preload cache. */ context_switch sched_migrate_task migrates process P to cpu-1

Process swapper/0 context switch (to process P) (uses mm_struct of Process P) switch_mm_irqs_off() switch_slb load_slb++ /* * load_slb becomes 0 here * and we evict an entry from * the preload cache with * preload_age(). We still * keep HW SLB and preload * cache in sync, that is * because all HW SLB entries * anyways gets evicted in * switch_slb during SLBIA. * We then only add those * entries back in HW SLB, * which are currently * present in preload_cache * (after eviction). */ load_elf_binary continues... setup_new_exec() slb_setup_new_exec()

                                    sched_switch event
                                    sched_migrate_task migrates
                                    process P to cpu-0

context_switch from swapper/0 to Process P switch_mm_irqs_off() /*

• Since both prev and next mm struct are same we don't call
• switch_mmu_context(). This will cause the HW SLB and SW preload
• cache to go out of sync in preload_new_slb_context. Because there
• was an SLB entry which was evicted from both HW and preload cache
• on cpu-1. Now later in preload_new_slb_context(), when we will try
• to add the same preload entry again, we will add this to the SW
• preload cache and then will add it to the HW SLB. Since on cpu-0
• this entry was never invalidated, hence adding this entry to the HW
• SLB will cause a SLB multi-hit error. */ load_elf_binary cont ---truncated---

Affected

• Linux >= 5434ae74629af58ad0fc27143a9ea435f7734410 < 01324c0328181b94cf390bda22ff91c75126ea57
• Linux >= 5434ae74629af58ad0fc27143a9ea435f7734410 < 2e9a95d60f1df7b57618fd5ef057aef331575bd2
• Linux >= 5434ae74629af58ad0fc27143a9ea435f7734410 < c9f865022a1823d814032a09906e91e4701a35fc
• Linux >= 5434ae74629af58ad0fc27143a9ea435f7734410 < b13a3dbfa196af68eae2031f209743735ad416bf
• Linux >= 5434ae74629af58ad0fc27143a9ea435f7734410 < 895123c309a34d2cfccf7812b41e17261a3a6f37
• Linux >= 5434ae74629af58ad0fc27143a9ea435f7734410 < 4ae1e46d8a290319f33f71a2710a1382ba5431e8
• Linux >= 5434ae74629af58ad0fc27143a9ea435f7734410 < 00312419f0863964625d6dcda8183f96849412c6
• Linux 4.20

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.